U.S. Pat. No. 6,806,942 proposed a so-called three-bulge two-waist projection objective system using a light source having a wavelength band with a relative width δλ/λ of larger than 0.002, or even larger than 0.005. In the system, the three bulges each have a positive refractive power and the two waists each have a negative refractive power. By selecting suitable materials, relatively high chromatic corrections are achieved, and the stability of the system is improved.
FIG. 1 together with FIG. 2 shows the projection objective provided by the U.S. Pat. No. 6,806,942, which includes 31 optical elements which can be divided into 5 groups: G1, G2, G3, G4 and G5. G1-G4 are illustrated in FIG. 1, wherein G1 is consisted of five lenses, namely the first to fifth lenses, among which, the first and third lenses are made of a high-transmittance material and the second lens is a negative lens; G2 is consisted of four lenses, namely the sixth to ninth lenses, all of which are negative lenses and the ninth lens is made of a high refractive index material; G3 is consisted of four lenses, namely the tenth to thirteenth lenses, all of which are positive lenses and the thirteenth lens is made of a high-transmittance material; G4 is consisted of the fourteenth to sixteenth lenses, all of which are negative lenses and are made of a high refractive index material. G5 is illustrated in FIG. 2, which is consisted of the seventeenth to thirty-first lenses, among which the twentieth lens is an optical flat and the twenty-second, twenty-seventh, thirtieth and thirty-first lenses are made of a high refractive index material. In FIG. 1, there is further indicated: an optical axis 107; an object plane 103; a central field point 113; an upper marginal beam 109 as well as a lower marginal beam 111 of the central field point; an outermost field point 121; and an upper marginal beam 115 as well as a lower marginal beam 119 of the outermost field point. Moreover, in FIG. 2, a system diaphragm 123 and an image plane 105 are further indicated.
Nevertheless, a projection objective with such design has a drawback that the system has a relatively poor telecentricity, especially on an object side. This causes the system to be extremely sensitive to the unevenness of a reticle surface, which unevenness may be generated during the fabrication of the reticle. As a result, after it is magnified by the projection objective, even a tiny convex or concave on the reticle surface may lead to a great change in the quality and in particular the distortion of an associated image formed on a wafer surface.